1. Technical Field
This invention relates to a method for producing a hole in a structure, and more particularly, to a method for accurately reproducing shaped holes.
2. Background Art
Airfoils, such as turbine blades and vanes, are typically exposed to high temperatures ranging from about 800xc2x0 C. to 1600xc2x0 C. One method of protecting airfoils from such extreme temperature conditions includes film cooling. Film cooling comprises the method of passing pressurized air through cooling holes, thereby allowing the air to pass over the exterior of the airfoil, as the airfoil rotates through the combustion gases. The geometric shape of the cooling holes includes both cylindrical holes and shaped holes. Cylindrical holes comprise holes generally having a circular cross section through the entire exterior wall of the airfoil, thereby allowing the pressurized air to pass through the airfoil. Manufacturing processes used to manufacture cylindrical holes are discussed in U.S. patent application Ser. No. 09/356,528, which is owned by the assignee of the present invention and hereby incorporated by reference.
Shaped holes, alternatively, include both a cylindrical meter section and a diffuser section. The cylindrical meter section allows the pressurized air to pass through the airfoil, and the diffuser section assists in directing the pressurized air over the airfoil""s exterior surface. In order to direct the pressurized air as close as possible to the exterior surface, the shape of the diffuser section is typically tapered from the cylindrical metered section to the airfoil""s exterior surface. The present method for manufacturing shaped holes is electro-discharge machining (EDM), wherein an appropriately shaped electrode contacts a structure that is typically immersed in a dielectric fluid. Near contact between the electrode and the structure, combined with a pulsed voltage, creates a spark between the electrode and the structure, thereby causing the structure to erode in the shape of the electrode.
EDM produces holes that accurately mimic the shape of the electrode. This machining process, however, often causes the tip of the electrode to deteriorate. If the tip of the electrode wears such that it fails to maintain its original configuration, the electrode may produce an undesirably shaped hole. Additionally, extended use of a single electrode often creates a sharp tip at the end of the electrode. The sharp tipped electrode, therefore, often contacts undesirable structural layers, thereby creating unwanted holes within the structure. For example, an airfoil is a two layer structure with a cavity therebetween. If the electrode loses its shape and obtains a sharp tip, the tip of the electrode may penetrate too deeply into the cavity and create a hole on the opposite side of the airfoil, thereby resulting in an additional and undesirable hole. One means of minimizing this potential risk includes frequent replacement of the electrodes. EDM, however, is an inherently time-consuming machining process, and frequent electrode replacement further increases such machining time. Furthermore, extended machining time and regular replacement of electrodes tends to increase the overall manufacturing cost.
What is needed is an efficient method for accurately manufacturing shaped holes in structures, such as airfoils.
The present invention utilizes pulsed laser beams to create both the diffuser section and the cylindrical meter section of a shaped hole, after which the diffuser section is milled. The pulsed laser beams include both unmodulated pulsed laser beams and modulated pulsed laser beams. An unmodulated pulsed laser beam (hereinafter referred to as xe2x80x9cunmodulated beamxe2x80x9d) typically has a pulse width of about 0.1 milliseconds (msec) to about 10 msec and a peak intensity on the order of about 1xc3x97106 W/cm2 to about 10xc3x97106 W/cm2. A modulated pulsed laser beam (hereinafter referred to as xe2x80x9cmodulated beamxe2x80x9d), typically has a pulse width of about 1 nanosecond (nsec) to about 500 nsec and a peak intensity greater than 1xc3x97108 W/cm2. For the purposes of this invention, an unmodulated beam and a modulated beam shall be defined in respect to each other. Specifically, a modulated beam shall be defined as having a shorter pulse width and higher peak intensity in comparison to an unmodulated beam, regardless of the pulse width and peak intensity of the unmodulated beam.
When a modulated beam contacts a structure, a majority of the material typically vaporizes, thereby creating a hole. A portion of this vapor, however, usually re-forms and adheres within and/or around the hole as re-solidified vapor. Alternatively, when an unmodulated beam is used to create a hole, the material typically melts and/or boils rather than vaporizes because an unmodulated beam has lower peak intensity in comparison to a modulated beam. The molten material often re-solidifies and adheres to the internal surface of the hole. More specifically, when manufacturing the cylindrical metered section of a shaped hole, the re-solidified material typically forms within the diffuser section.
Although the re-solidified material may have similar characteristics as the parent material, the inventor of the present invention has recognized that it is less difficult to machine the re-solidified material in comparison to the parent material. The present invention, therefore exploits this distinction by first creating an undersized diffuser section using pulsed laser beams. Pulsed laser beams thereafter create the cylindrical meter section. Any re-solidified material that forms within the diffuser section is later milled away using a mechanical milling tool, thereby creating the desired shape of the diffuser section.
Accordingly, the present invention relates to a method for producing a shaped hole in a structure by initially creating a cavity in the structure. The cavity serves as an undersized diffuser section and is formed by directing a first pulsed laser beam, at a first angle, toward the structure. The first pulsed laser beam can be a modulated beam or an unmodulated beam, but is preferably a modulated beam because it typically creates a more accurate cavity. A second pulsed laser beam is thereafter directed toward the structure at a second angle in order to form the cylindrical meter section of the shaped hole. The second angle may or may not be equal to the first angle. The second pulsed laser beam can be a modulated beam, an unmodulated beam or a combination thereof It is preferable, however, to utilize an unmodulated beam followed by a modulated beam. The unmodulated beam first removes a portion of the structural material, thereby forming a guide hole for the modulated beam to follow when completing the cylindrical meter section. Utilizing both an unmodulated beam and a modulated beam in this order exploits the advantages of each type of beam. Specifically, forming a guide hole utilizing the unmodulated beam quickly removes a significant portion of the structural material because the unmodulated beam removes the material in the form of droplets, which is faster than removing the material in the form of vapor. The modulated beam thereafter cleans the hole by removing additional structural material by vaporization. Removing a significant portion of the material with the unmodulated beam, before using the modulated beam, reduces the amount of material to be later removed by the modulated beam, thereby reducing the amount of material that will be removed by vaporization. Decreasing the amount of structural material to be removed by vaporization minimizes the potential that such vapor could potentially re-solidify within or around the hole, thereby increasing the accuracy of the cylindrical meter section of the shaped hole. Any re-solidified material formed in the diffuser section during the laser drilling process is removed by milling such re-solidified vapor.
Manufacturing shaped holes utilizing pulsed laser beams and a milling tool is less time consuming than an EDM process, which translates into higher productivity and reduced operating costs. Additionally, utilizing pulsed laser beams in lieu of electrodes minimizes the material cost associated with manufacturing shaped holes because the lasers do not require replacement, unlike the electrodes. The process of the present invention also produces more accurately shaped holes compared to those produced by the EDM process. The savings associated with manufacturing time and material costs, in conjunction with the improved quality of the shaped holes, therefore, make the method of the present invention an attractive alternative manufacturing technique for the production of shaped holes.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.